feat(cache,worker): overhaul cache layer and add generation task lease recovery

- shared/cache: add Options/L1/async/metrics/prefix decorators, multi-key ops, Redis pool tuning, and JSON readthrough metrics
- worker-generate: claim tasks via DB lease + heartbeat, requeue stale queued tasks, expire dead leases with refund/cache invalidation
- tenant: version article cache keys so worker recovery invalidations propagate cleanly
- shared/config: expand Redis (pool/timeouts/TLS) and Generation (lease/recovery) configs with defaults

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/internal/shared/cache/cache.go

@@ -8,9 +8,18 @@ import (
 
 var ErrNotFound = errors.New("cache: key not found")
 
+type Item struct {
+	Key   string
+	Value []byte
+	TTL   time.Duration
+}
+
 type Cache interface {
 	Get(ctx context.Context, key string) ([]byte, error)
+	GetMulti(ctx context.Context, keys []string) (map[string][]byte, error)
 	Set(ctx context.Context, key string, value []byte, ttl time.Duration) error
+	SetMulti(ctx context.Context, items []Item) error
 	Delete(ctx context.Context, key string) error
+	DeleteMany(ctx context.Context, keys []string) error
 	DeletePrefix(ctx context.Context, prefix string) error
 }